[PATCH] D129636: Fix a LSR debug invariance issue

[Markus Lavin via Phabricator via llvm-commits]

markus added a comment.

In D129636#3648003 <https://reviews.llvm.org/D129636#3648003>, @jmorse wrote:

> Thanks for hunting this down, caching layers affecting codegen sounds hard to debug.
>
> I'm no SCEV expert, so a couple of possibly obvious questions:
>
> - Is it safe to store the SCEV* for a Value that's then forgotten?
> - Is there a risk that the presence of debug-info will now cause a SCEV to be forgotten that _should_ have been remembered, because of unconditionally forgetting a SCEV if it's used by debug-info?

Unsure and possibly :) I am hoping that the other reviewers will bring some clarity to these questions.

For the first question about storing a SCEV I know there was a lot of discussion about that when I did the original patch some years ago (D87494 <https://reviews.llvm.org/D87494> and D91711 <https://reviews.llvm.org/D91711>) but since then that was fixed and it has afterwards been completely rewritten so not sure what the status is now.

For the second point maybe `ScalarEvolution::getExistingSCEV()` could be used to see if this SCEV existed before analyzing the `dbg.value` or not so that we don't end up forgetting about values that we would not forget about hadn't it been for the `dbg.value`.


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